Wave expansion arrangement for cathode-ray oscilloscopes



Oct. 9, 1945.V

J. L.. THEISEN WAVE EXPANSION ARRANGEMENT FOR CATHODE RAY OSClLLSCOPES Filed Aug. 1, 1941 2 Sheets-,Sheet 1 DE@ lA Ihvehtpr. Joseph L. Theisen, b WM? g Hi J. l. THElsEN 2,386,728

WAVE EXPANSION ARRANGEMENT FR CATHODE RAY OSCILLOSCOPES Oct. 9, 1945.

Filed Aug. 1, 1941 2 sheets-sheet 2 Inventor z Joseph L. Theisen,

His Attorney.

atented et. 9, 45

WAVE

CAODE-RAY OSSSOFE Joseph L. '.llheisen, Marblehead,

SION ARRANGEMEN'E )F0 assigner to General Electric Company, Y orporation et New York Application August 1, 19411, Serial No. 404,967

3 Claims.

My invention relates to oscillographic devices of the visual or recording type and concerns particularly Oscilloscopes of the type having a signaltracing beam, such as cathode ray Oscilloscopes, with means for deecting the beam in a given plane in response to a signal wave to be examined in the oscilloscope and for deiiecting the beam in a transverse plane in response to a time-axis or sweep wave.

It is an object of my invention to Provide methods and apparatus for expanding on the screen of the oscilloscope an image of a portion of the signal wave to be observed, and to select any desired portion of the signal wave for expansion.

It is a more specific object of my invention to provide methods and apparatus for modifying the shape of the sweep wave of the oscilloscope and to introduce a portion of greater steepness fo expansion of the wave. l

Other and further objects and advantages will become apparent as the description proceeds.

'I'he sweep-wave circuits and wave expansion arrangements described in my application are modifications of those disclosed in the copending applications of Ellsworth D. Cook, Serial No.-

338,360, tiled June 1, 1940, and Harris A. Thompson, Serial No. 350,409, filed August 3, 1940, both assigned to the same assignee as the present application.

For convenience and simplicity the invention is described in connection with cathode ray oscilloscopes having deflection plates for electrostatic deiiection of the beam. However, my invention is not limited to this particular form of apparatus.

In carrying out my invention in its preferred form I utilize discharge tube circuits for producing a sweep wave for application to the sweep deiiection plates of the oscilloscope, 'I'he basic portion of the sweep-wave circuit may be similar to the basic portion -of the sweep-wave circuits of the aforesaid applications. However, I produce expansion of the wave by greatly amplifying the normal triangular or sawtooth wave of the conventional sweep-wave circuit, so that but a small portion of the amplified wave represents full deiiection in the direction of the time axis of the oscilloscope screen which is usually the horizontal axis. In order to prevent the cathode ray beam from striking the sides of the tube or the anode plates and thus overloading the beam energizing circuit, I provide means for suppressing the portion of the sweep wave beyond that utilized for producing full scale deiiection of the cathode ray beam. For this purpose I may either cut oil the excess portion of the sweep wave or suppress the beam itself during the unwanted portions of the sweep wave. The signal wave, the shape of which is to be represented on the cathode ray screen, is applied to signal deection producing or vertical deection plates oi the cathode ray tube. For selecting the portion of the signal wave which is to be expanded or spread across the cathode ray screen, I provide a phase-shifting circuit arranged to vary the phase relationship between the high speed sweep wave and the signal wave.

My invention will be understood more readili7 from the following Idetailed description when considered in connection with the accom ,--t zi :f x drawings and those features oi the invention which are believed to be novel and patentable are pointed out in the claims appended hereto.

In the drawings Fig. 1 is a block diagram schematically representing a sweep-wave circuit which constitutes one embodiment of my invention and which represents diagrammatically the shapes of waves appearing in various portions of the circuit; Fig. 2A is a graph representing a conventional form of square wave from which a sawtooth sweep wave may be produced. and Fig. 2B is a graph representing such a sawtonth wave; Figs. 2C and 2D are graphs representing modiiied sweep waves which may be obtained from the sawtooth wave of Fig. 2B in accordance with my invention for accelerating the sweep speed during a portion of the sweep-wave cycle; Fig. 3 is a block diagram in form similar to Fig. 1 representingaI modified sweep-wave circuit forming another embodiment of my invention; Fig. 4 is a circuit diagram schematically representing actual electrical circuits and elements of the apparatus represented by the rectangles captioned "Ampliiier" and Blanking circuits in Fig. 3, and Figs. 5 and 6 are circuit diagrams of tvgo dierent clipper circuits which maybe employed. Like reference characters are used throughout the drawings to designate like parts.

As in the apparatus described in the aforesaid Cook and Thompson applications, the sweep wave is synchronized with a signal voltage by tapping oi a portion of the signal wave or a voltage proportional to it, and applying such voltage, as a synchronizing voltage, to input terminals of the sweep-wave circuit. Accordingly, a pair of synchronizing terminals Il is provided to which is applied the input signal I2 or a voltage representing it. The basic portion of the sweep-wave circuit consists of a phase shifter I3, a synchronizing ampliiier or buiier I6, a square-wave senerator Il, and a sawtooth circuit IB connected to the output of the square-wave generator Il for converting the square wave into a triangular wave. The foregoing elements may be conven-I tional in form, and do not in themselves constitute my invention.

In order to increase the speed of the sweep wave y I provide an amplifier I 1, the input of which 'is connected to the output side of the sawtooth circuit Il. In the embodiment of my invention illustrated in Fig. 1, a clipper circuit Il is provided for cutting of! all but the lower portion of the amplified sawtooth wave produced by the amplifier I1. In the arrangement illustrated the output of the clipper circuit I8 is applied to a pair of electrostatic deection plates I of a cathode ray tube 20 and the sweep wave is a voltage wave. However, my invention is not limited to use with oscillographic devices of the type having electrostatic sweep instead of electromagnetic sweep, for

example, wherein deflection coils are used carryhorizontal deflection plates I9. However, if it is desired to superimpose an image of an expanded section of the signal wave onl the same .cathode ray screen with an image of the complete signal wave, the high speed sweep wave from the clipper Il may be passed through an addition circuit 23 for combination with a regular speed sweep wave proyided by a regular sweep-wave generator 24 as in the case of the aforesaid Cook application.

Any suitable form oi' phase-shifting circuit may be employed for the phase shifter Il. It may take the form, for example, of a variable width squarewave generator including a suitable adjusting device such as a variable condenser or rheostat for changing the ratio between-the time duration ofv the positive and negative portions of the square wave, and ln which the abruptly rising portions of the wave are synchronized with the signal voltage applied at the terminals Il and in which the abruptly falling portions of the wave serve to synchronize the wave initiated in the Square-wave generator I5. 'I'he buffer I4 consists of a suitable amplifier circuit of a type well known to those skilled in the radio frequency art. The variable width square-wave generator II may consist of any suitable device for producing a lsquare wave and may have a circuit similar in principle of operation to that ofthe phase shifter I3.

\The amplifler l1 may be a vacuum tube amplifler oi' any suitable type and the clipper il may be one of several different types according to the type of wave to be produced. For producing a wave with the top portion suppressed, lsuch as the wave 3l of Fig. 2C, the clipper circuit may take the form, for example, of a vacuum tube circuit suchl as illustrated in Fig. 5. If the clipper il is to produce a wave I8, such as that illustrated in Fig. 2D, it may take the form of the circuit illustrated in Fig. 6. The circuits mentioned by way of example or other suitable circuits for the purposes 'mentioned may be employed.

The clipper circuit illustrated in Fig. 5 comprises a pair of three-element vacuum tubes 1I and 14. The tubes have anodes 1I and 1l rebuffer M.

erator 83 is selected to provide a square wave of spectively connected to a source oi.' anode potential B plus through conventional anode resistors. For stability cathode resistors may also be. provided. The tube 13 serves as van inverter having a control electrode 11 capacitatively coupled to the output of the amplifier l1. The tube 1I, in turn, which is the clipper tube, has a control electrode 1l capacitatively coupled to the anode 1I of the tube 1L For the clipper tube 1l a vacuum tube is chosen in which the current is cut of! by a negative control-electrode voltage of the value M, or the cutof! point of the'clipper tube 14 may be adjusted to the desired value such as the value M, by providing an adiustable'source of biasing voltage 1l. Said voltage source may take the i'orm of a potentiometer connected across a battery of cells. The tube 1I serves to invert the amplified triangular wave I! to form an inverted wave I2. in the manner of conventional resistance-condenser coupled circuits. When the inverted, steep triangular wave $2 is applied to the control electrode 1l of the clipper tube 14 the wave is again inverted but the portion of the wave exceeding the value M is cut oil.' because the control electrode 1l has no further control over the tube 14 when the voltage M is exceeded. Thus the discharge current in the tube 14 remains at zero and the anode voltage remains at the maximum value, representing the cutoff value M to form a wave of the shape represented by the curve Il. This voltage wave appears between the output conductors l0 and Il.

y The cuto point may be adjusted by means of the and to clip at a higher voltage when the bias is decreased.

If it is desired that the sweep voltage shall drop to zero after the steep portion I4 of the sweep wave to form a wave of the shape represented by the curve I8, the clipper circuit of Fig. 5 may be modified as represented by Fig. 6. By way of illustration a double triode 82 is shown having a left-hand triode element 1I' and a right-hand trlodeelement 14 corresponding to the triodes 1I and "of Fig. 5. 'I'he triode element 13' has its electrodes connected in the manner illustrated in Fig. 5to form an inverter for the output of the amplifier I1. The control electrode 1l' of the triode element 14' is also capacitatively connected to the anode of the triode element 1l and an adjustable biassource 1l may again be provided. In the arrangement of Fig. ,6, however, the anode potential for the anode 18' is not provided by a fixed voltage source but instead by a suitable source of square-wave potential. For example, a narrow width square-wave generator 8l may be provided which is synchronized with the sweep wave by a connection to the output side of the The narrow width square-wave gennarrower width than the wave il provided by the generator I5, the width of the wave preferably 'being no greater than the length of the projection on the horizontal axis of the steep portion I4 of the sweep wave.

During the positive portion of the-square wave Il provided bythe narrow width square-wave generator the triode element 13' will function in the same manner as the conventional resistance-coupled amplifier to invert the triangular wave 32 and form the inverted wave 32'. The triode element 14' will likewise function as a resistance coupled amplifier to reinvert the triangular wave 32' until the cutoff point M is reached, thus tracing the steep portion Il of the sweep wave. Thereupon the anode voltage of the triode element 14' falls abruptly to the negative value of the square wave 84 and the sweep wave drops to a uniform negative value as represented in the curve 36. Accordingly a sweep wave of the shape represented by the curve 36 appears at the output terminals 80 and 8l.

The variable width square wave produced by the phase shifter I3 is represented schematically at 29 belowthe phase shifter I3. The square Wave produced by the square-wave generator I5 is represented schematically at 30. If it is desired to make the amount of expansion adjustable, the square-wave generator I5 will also be so constructed as to include means for adjusting the width of the wave 30. The sawtooth Wave produced by the sawtooth circuit I6 is represented at 3l and will be seen to have rising and falling portions corresponding to the positive and negative portions of the rectangular wave 3U. After passing through the amplier I1 the 'sawtooth triangular wave 3l appears'in a greatly amplined form as represented schematically at 32 in which the rising steep portion 33 of the wave 32 is greatly increased in slope for rapidly switching the cathode ray beamacross th'escreen of the tube. It will be understood of course that the curves 3| and 32 are simply schematic and are not intended to represent the actual or most desirable ratio of ampliilcation.

As indicated in Fig. 2B, the curve 32 is ampliiied to such an extent that a very small portion 34 of the`entire wave, viz: that from the base Whether the shape of the high speed sweep wave is such as shown-in the curve 35 of Fig. 2C

line to the dotted line ,M represents full horif zontal deection of the cathode ray beam. In order to prevent injury to the tube or overloading which would occur from allowing the beam to be driven against the side of the tube. the portion of the wave 32 above the horizontal line M is suppressed. vThis is accomplished by means of the clipper I8 which simply cuts oi all of the wave above the line M to form a curve of the shape represented by Fig. 2C or/Fig. 2D. The ratio of expansion produced by the ampliiication Y of the triangular sweep wav 3| is represented by the ratio between the dis nces a and b. 'The period of the sweep wave as represented by a and b is the length measured in time units of the projection of the steep portion 34 of the clipped sweep wave 35 or 36.

If the high speed sweep wave 35 is`combined with anormal speed wave by means of the addition circuit 23, the appearance of the result-r l ant sweep wave willbe substantially as represented at 31, with thehigh speed wave 38 superimposed on a slow speed triangular sweep wave 38 produced Vby the regular sweep-wave circuit 24. The high speed portion 39 of the wave 31 corresponds to the steep portion 34 of the wave 35 or 36 and its relationship with the remainder of the wave 31 may be varied by adjustment of the phase shifter I3. Thus, in effect. the portion 39 of the sweep wave is made to move back and forth along the line 31 by adjustment of the phase shifter I3. If a high speed sweep wave or an expansion circuit only is employed. the selection of the portion to be expanded takes place in the same manner since adjustment of the phase shifter I3 changes the phase relationship be tween the signal wave and the steep portion 34 of the sweep wave 35 or 33. If desired the unused, less steep negative slope portion 48 of the sweep wave 35 may also :be eliminated inany siitable manner as by means of a iiy-bacl-r cuto e: in the curve 35 of Fig. 2D, expansion will take place in the same manner since the high speed portions 34 in the two shapes oi. sweep wave correspond.

' A modied sweep-wave expansion circuit is represented in Fig. 3, employing elements I3 to I,1 inclusive, which may be of the same type as3- the correspondingly numbered elements in the` embodimentof Fig. 1. The clipper I8 of Fig. 1 is, however, omitted and instead a blanking circuit is` provided for the purpose of cutting off' the cathode ray beam of the oscilloscope tube 2,0 when the sweep wave 32 reaches a voltage rep-l resented by the horizontal line M. The ampliiler l1 may consist of'one or more vacuum tube stages with a voltage tapped from one of the stages to operate the blanking circuit 45. The arrangement may be such as that illustrated' in uum tube 46 is provided to serve as the last two stages of the amplifier I1. A triode vacuum tube 41 serves as the controlling element of the blanking cir'cuit 45.

The tube 46 comprises a pair of anodes 48 and 49 to which a source of anode voltage 50 is connected through'a pair of anode resistorsi5l and' 52, 'the mid-points of which are grounded through ripple-absorbing veondensersl There is a common cathode 53 which is grounded, and there are grids or control electrodes 54 and.55

' associated with the anodes 48 and 49 respectively.

The grid 54 is capacitatively coupled to the output of the previous stage of the amplier or, if the amplifier consists of only two stages, to the conductor 53, to which the positive output of the sawtooth circuit I6 is connected. The grid 55 is capacitatively coupled to the anode 48 in accordance with the well-known resistance capacity amplier coupling. The ampli-fier output v appears at the anode 43 and is applied through a conductor 51 to one of the horizontal deflection plates I9 of the cathode ray tube 20. It will be understood that the other horizontal deflection plate is either grounded or held ata fixed potential by a suitable device such as a potentiometer in accordance with the Well-known cathoderay oscilloscope practice. y

In order to suppress the cathode ray oscilloscope beam when the voltage of the sweep .wave 32 exceeds a value corresponding to full scale defiection across theoscilloscope screen, the control electrode or grid 58 of the oscilloscope tube 20 is connected through a conductor 59 to the output of the blankingcircuit 45. As shown in Fig. 4, the blanking circuit includes the tube 41 having an anode 68 to which the output conductor 59 is connected, a control electrode or grid 5I, and a cathode 32. y

'I'he discharge tube 41 'may advantageously be .a vacuum tube of the type having a sharp upward bend in the characteristic curve representing the relationship between its anode current plotted along a vertical axis and its grid voltage plotted along a horizontal axis. The grid 6I is capacitatively coupled to the output terminal of the ampliler I1, which is the anode 49 of the tube 46. In order to cause the blanking tube 41 to operate near the bend in its characteristic curve, an adjustable negative bias is applied thereto. This may take the form of a'potentiometer 63 connected across a voltage source 64 connected o n the positive side to the cathode 62. In order that the blanking tube 41 mayvsupply an adequate negagrounded.' The blanking-tube anode 60 is connected to the positive side of a source of anodel voltage, such as the source 56, through an anode resistor 66 which also has its mid-point bypassed through a ripple-absorbing condenser connected to ground.. Th constants of the circuits, the characteristics of the tube 41 and -the bias voltage provided by the grid bias potentiometer 63 are so chosen as to cause the blanking tube 41 to become strongly conducting, when the voltage of the wave 32 appearing at the amplifier anode 49 reaches the value represented by the horizontal line M. Since the tube 41 is one having the aforesaid sharp bend characteristic, as long as the sweep-wave voltage remains below the value M, the anode current of the tube 41 is negligible and a positive voltage is maintained on the cathode ray oscilloscope grid 56. However, when the sweep-wave voltage exceeds the value M, the bend in the characteristic curve of the tube 41 is passed and a relatively heavy discharge current flows through the tube 41 causing a large voltage drop in the anode resistor 66, depressing the voltage of the oscilloscope grid 56 below that of the oscilloscopecathode 65 and cutting off the cathode ray beam to suppress the wave traced on the screen. It will be understood that suitable means, adjustable if desired, are ordinarily provided for normally maintaining the potential of the oscilloscope grid 58 at the value which produces the most satisfactory operation. This may take the form of a voltage divider, for example. Such a voltage sistor 66 and a rheostat 61 of suitable value connected between the anode 60 and ground. Thus normally the potential of the oscilloscope grid 58 is fixed by the ratio between the resistance of the rheostat 6l and the resistance 66 which are connected in series to'the voltage source 50. However, when the blanking tube 4l becomes strongly conducting it draws current of such magnitude through the anode resistor 66 as to depress th grid voltage of the oscilloscope 58,. The approximate shape of the voltage wave applied to the grid 58 is represented by the curve 68 as shown below the-rectangle representing the blanking circuit 45. It will be observed that for the time duration of the steep. portion 3! of the curve S2, the curve 66 remains positive as represented by the positive loop 65). However, when the sweep-wave voltage exceeds Vthe value M the anode voltage of the blanking tube 4l drops abruptly along the line I but remains strongly negative with respect to the oscilloscope 66 until the sweep voltage falls again to the value M. The exact shape of the negative portion 1I of the curve 68 will of course depend on the characteristics of the tube 41.

I have herein shown and partially described certain embodiments of my invention and cer-A wave is produced by the uninterrupted the purpose of explaining its practice and show- 'i-ng its application, but it will be obvious to those skilled in the art that many modifications and variations are possible, and I aim therefore to cover all such modifications as fall within the scope of my invention which are defined in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A signal wave expansionarrangement for a sweep-wave circuit of an oscillographic device having a signal-tracing beam comprising in combination with a sawtooth wave generator for producing a sweep wave for detlecting a signal-tracing beam along a sweep axis of the oscillographic device and spreading an image of the signal wave on the screen of the oscillographic device, an amplifier for increasing the amplitude of the sweep wave and thus increasing its slope to increase the speed of sweep, an/ a device responsive to the sweep wave at a value thereof exceeding the amplitude required to produce-full scale deflection along the sweep axis of the oscilloscope for interrupting further sweep of the signal-tracing beam, whereby an expansion of the traced signal portion of the amplified sweep wave.

.divider is represented in Fig. 4 by the anode required for full scale deflection, a clipper circuit connected to said sawtooth wave generator for cutting off the portion of the sweep wave which would otherwise have an amplitude greater than that required to produce a deflection the full length of the desired sweep axis of the device whereby the slope of the used portion of the sweep wave is caused to be relatively great to produce high speed sweep and expansion of a signal wave to be examined in the device during a portion of thecycle of the signal wave to be examined.

3. An oscilloscope sweep-wave generator comprising generating means for producing a triangular wave for application to one of the deflection circuits of a cathode ray oscilloscope but of greater peak value than required for full scale deflection, said oscilloscope being of the type having a control electrode or grid circuit, a blanking circuit responsive to the magnitude of the sweep wave having output terminals for connection to the control electrode or grid circuit of the oscilloscope for applying a negative voltage therein of sufficient value to cut off the cathode ray beam whenever the sweep wave exceeds the amplitude required to,sweep the cathode ray beam the full desired vlength of the sweep axis of the oscilloscope, whereby the slope of the used portion of the n sweep wave is relatively great to produce high speed sweep and the signal wave existing during only a portion of the cycle is expanded to the full length of the sweep axis of the oscilloscope.

JOSEPH L. THEISEN. 

